Water quality management is of particular concern in the cultivation of aquatic organisms for such purposes as food, recreation, education, research and hobbies. Fish and other aquatic animals require an adequate supply of oxygen in order to ensure survival. In addition, aerobic bacterial processes require an adequate supply of oxygen to convert toxic waste into nontoxic by-products. It is crucial to maintain a minimum level of dissolved oxygen in water in order to maintain this aquatic life.
When water is free of all aquatic life, at a water temperature of approximately 70.degree. F. (21.1.degree. C.), the dissolved oxygen level in water is approximately 9 parts per million (ppm). However, in the presence of aquatic animals and accompanying aerobic bacterial processes, the oxygen level may become insufficient to support such life. Therefore, it is desirable for aquaculturalists to have a simple, reliable method for testing the concentration of dissolved oxygen in a water sample in order to ensure an adequate concentration of dissolved oxygen for maintaining such aquatic life.
The prior art discloses various methods for determining the concentration of dissolved oxygen in water. For example, the Winkler or Iodometric method uses titration based upon the oxidizing property of dissolved oxygen to determine the concentration of dissolved oxygen in water. The electrometric method is based upon the rate of diffusion of molecular oxygen across a membrane of an electrode. However, these methods are prohibitively expensive both in terms of time and money to the home aquaculturalist as they require specialized titration equipment and solutions or membrane electrodes and complicated procedures for use.
Another prior art method for determining the concentration of dissolved oxygen in water ranging from 0 to 13.0 mg O.sub.2 /L uses a photometer to compare the color of a test water sample with a control sample containing a known concentration of oxygen. An ampul having a fracturable tip and containing an undisclosed reagent is immersed into a water sample. When the tip is fractured, a vacuum created within the ampul draws a predetermined volume of water into the ampul. The water and reagent are mixed within the ampul and the ampul is placed in the photometer to determine the concentration of dissolved oxygen in the water sample. In the presence of dissolved oxygen, the color of the reagent within the ampul changes from yellow to purple. The intensity of the purple color is proportional to the concentration of dissolved oxygen in the sample.
This method suffers from a number of drawbacks in terms of accuracy, convenience, and cost of use, especially since specialized equipment is needed. With regard to accuracy, the results obtained by use of a photometer are influenced by bubbles or particulate matter suspended in the water sample. The test results may also be influenced by the depth and turbulence of the water in the area from which the sample is taken, as well as temperature, light, sludge deposits, microbial action, travel time, mixing, and other factors affecting the water sample.
The prior art discloses another method which is used to determine concentrations of oxygen in water, such as boiler feedwater, ranging from 0 to 800 micrograms of oxygen per liter. This method follows substantially the same steps as the immediately preceding method except that a different reagent is used. In the presence of dissolved oxygen, the reagent color changes from yellow to blue, the depth of the blue color being proportional to the concentration of dissolved oxygen. This method suffers from the same types of drawbacks as that discussed above. Moreover, sodium hydrosulfite in the sample may reduce the oxidized form of the indicator solution and seriously interfere with the results of the test. Also, a 100,000-fold excess of hydrazine may reduce the oxidized form of the indicator solution.
Another method for determining dissolved oxygen concentration for use in aquaculture is disclosed in the prior art which involves substantially the same test procedures as those followed in the latter two aforementioned methods, except that the reagent changes color from yellow to purple in the presence of dissolved oxygen. The intensity of the purple color is proportional to the concentration of dissolved oxygen in the sample. This method suffers from similar drawbacks to those discussed above. In addition, magnesium, which is commonly present in sea water, may cause interference with the test results.
The prior art discloses a number of methods for determining the presence of oxygen in a gas.
There is disclosed in the prior art an oxygen-absorbing agent comprised of a ferrous compound, and, in a preferred embodiment, a metal powder such as a reduced iron, the composition exhibiting a color change upon absorption of oxygen.
The prior art also discloses a process for detecting the presence of oxygen in a gas by contacting the gas with a composition comprising a supported oxide of a transition metal of Group VB or VIB of the Periodic Table in a lower valence state. Oxygen present in the gas effects a color change in the composition. A process for detecting oxygen in a gas stream by contacting the gas with a reaction product formed by the reduction of an organo-transition metal compound with a metal hydride or an alkyl compound of metal of Groups IA, II, or IIIA of the Periodic Table is also disclosed in the prior art. The resulting product contains the transition metal in a lower valence state, and undergoes a color change.
The prior art discloses the use of ammoniated copper to indicate by color change the amount of oxygen in a gas mixture. A gas mixture containing oxygen is passed over a packed bed of inert particulate support material coated with ammoniated copper. The copper reacts with the oxygen to produce copper oxide or copper hydroxide, which in turn reacts with the ammonium ion to produce the blue cuprammonium ion. The length of the resultant colored stain on the coated support material is proportional to the amount of oxygen passed over the support material.
A number of methods for determining the concentration of oxygen in liquid are disclosed in the prior art.
There is disclosed in the prior art the use of a tube containing a colorimetric reagent which is partially evacuated and has a readily frangible tip. Examples of suitable colorimetric reagents include 5,5-indigo disulfonic acid and potassium thiocyanate. When the tube is fractured, a predetermined quantity of liquid is drawn into the tube. The reagent reacts with the liquid and develops a color proportional to the concentration of the material to be determined in the liquid. A product based upon this prior art is commercially available for use by fresh water and marine aquarists in which the dissolved oxygen content in a water sample is proportional to the intensity of the resulting blue color of the reagent within the ampul. An oxygen level of between 0 and 10 ppm is ascertainable upon visual comparison of the color of the reagent with a color chart.
The prior art discloses a method and means for determining the dissolved oxygen content in water based on the assertion that the amount of dissolved oxygen is inversely proportional to the iron content of the water. A color indicator containing tannic acid or other reagent capable of detecting minute quantities of ferrous iron in water changes color when the tannic acid or other reagent reacts with dissolved ferrous iron in the water.
Another prior art compound for indicating the presence of oxygen in a gas is comprised of at least one dyestuff, at least one alkaline substance selected from the group of oxides or hydroxides of alkaline earth metals, aluminum hydroxide, phosphates, carbonates, or organic acid salts of alkaline earth metals and at least one reducing agent selected from the group consisting of dithionites, ferrous compounds, reducing saccharides, and mixtures thereof.
While the prior art discloses a variety of methods for determining the dissolved oxygen concentration in liquids or gases, a simple, reliable method for determining the concentration of dissolved oxygen in a water sample capable of use by the home aquacultural enthusiast is needed.